Organic Chemistry Exam Study Guide: Conformations, Stereochemistry, and Radical Reactions

Conformations of Cyclohexane and Substituted Cyclohexanes

Chair Conformations

The chair conformation is the most stable form of cyclohexane due to minimized torsional and steric strain. Substituents on cyclohexane rings can occupy either axial or equatorial positions, which affects their stability and reactivity.

• Axial positions: Oriented perpendicular to the plane of the ring (up or down).

• Equatorial positions: Oriented roughly parallel to the plane of the ring, extending outward.

• Stability: Bulky substituents prefer the equatorial position to minimize 1,3-diaxial interactions.

• Example: In cis-1,3-dimethylcyclohexane, the lowest energy chair conformation has both methyl groups in equatorial positions.

Key Formula:

$\text{Number of axial hydrogens} = 6$ (for unsubstituted cyclohexane)

Substituent Effects on Stability

Substituents on cyclohexane rings influence the stability of different conformers.

• Bulky groups: Prefer equatorial positions.

• Energy difference: The energy difference between axial and equatorial positions increases with the size of the substituent.

• Example: In 1,3-dimethylcyclohexane, the conformation with both methyl groups equatorial is most stable.

Stereochemistry: Isomers, Diastereomers, and Enantiomers

Types of Isomers

Organic molecules can exist as different isomers, which are compounds with the same molecular formula but different arrangements of atoms.

• Constitutional isomers: Differ in connectivity of atoms.

• Stereoisomers: Same connectivity, different spatial arrangement.

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

Example: (2R,3S)-2,3-dibromo-3-chloropentane and (2S,3R)-2,3-dibromo-3-chloropentane are diastereomers.

Assigning Stereochemistry

Stereochemistry is assigned using the Cahn-Ingold-Prelog (CIP) priority rules.

• R/S configuration: Assign priorities to substituents, orient the molecule, and determine the configuration.

• Example: D-Threonine (2R,3S) configuration is determined by assigning priorities to the groups attached to each stereocenter.

Key Formula:

$\text{Specific rotation: } [\alpha] = \frac{\alpha_{\text{obs}}}{l \cdot c}$ Where $[\alpha]$ is the specific rotation, $\alpha_{\text{obs}}$ is the observed rotation, $l$ is the path length in decimeters, and $c$ is the concentration in g/mL.

Radical Reactions: Halogenation and Stability

Free Radical Halogenation

Free radical halogenation is a reaction in which alkanes react with halogens (e.g., Cl2, Br2) to form alkyl halides via a radical mechanism.

• Initiation: Formation of radicals (e.g., Cl2 → 2 Cl•).

• Propagation: Radicals react with alkanes to form new radicals and products.

• Termination: Two radicals combine to form a stable molecule.

• Example: Chlorination of 3,4-dimethylhexane produces several monochlorinated products; the major product is formed via the most stable radical intermediate.

Key Equations:

$\Delta H = \text{Bond dissociation energy of reactants} - \text{Bond dissociation energy of products}$

Radical Stability

The stability of carbon radicals increases with the degree of alkyl substitution.

• Order of stability: tertiary > secondary > primary > methyl

• Allylic and benzylic radicals: More stable due to resonance stabilization.

• Example: (CH3)3C• is more stable than CH3•.

Alkyl Halides: Structure, Reactivity, and Nucleophilicity

Classification of Alkyl Halides

Alkyl halides are classified based on the carbon to which the halogen is attached.

• Primary: Halogen attached to a carbon bonded to one other carbon.

• Secondary: Halogen attached to a carbon bonded to two other carbons.

• Tertiary: Halogen attached to a carbon bonded to three other carbons.

Example: CH3CHClCH3 is a secondary alkyl halide.

SN2 Reactivity

SN2 reactions are bimolecular nucleophilic substitution reactions. The rate depends on both the nucleophile and the substrate.

• Best substrates: Primary alkyl halides react fastest via SN2.

• Steric hindrance: Tertiary alkyl halides are poor SN2 substrates.

• Example: 1-iodobutane reacts faster than 1-fluorobutane in SN2 reactions with NaCN.

Key Equation:

$\text{Rate} = k[\text{Nucleophile}][\text{Alkyl Halide}]$

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile.

• Strong nucleophiles: CN-, I-

• Weak nucleophiles: BF3, H2O

• Example: CN- is a stronger nucleophile than H2O.

Bond Dissociation Energies and Reactivity

Bond Strengths in Alkyl Halides

Bond dissociation energy (BDE) is the energy required to break a bond homolytically.

• Lower BDE: Indicates a weaker bond, more reactive in radical reactions.

• Example Table:

Compound	BDE (kcal/mol)
CH3-Br	68
(CH3)2CH-Br	66
(CH3)3C-Br	65

Conclusion: The carbon-bromine bond is weakest (lowest BDE) in tertiary alkyl bromides.

Carbocation Stability and Substituent Effects

Allylic Stabilization

Allyl substituents stabilize carbocations through resonance and inductive effects.

• Resonance: Delocalization of positive charge over multiple atoms.

• Inductive effect: Electron donation from adjacent groups stabilizes the cation.

• Example: Allyl carbocation is more stable than a simple alkyl carbocation.

Application: Predicting Products and Drawing Structures

Monochlorination Products

Chlorination of alkanes can yield multiple products depending on the position of substitution.

• Major product: Formed via the most stable radical intermediate.

• Example: Chlorination of 3,4-dimethylhexane yields several monochlorinated products; the major product is from tertiary carbon substitution.

Organic Reaction Mechanisms

Understanding the mechanism allows prediction of major products.

• Example: Reaction of 2-bromopentane with NaCN yields 2-cyanopentane via SN2 mechanism.

Summary Table: Types of Isomers

Type	Description	Example
Constitutional Isomers	Different connectivity	Butane vs. isobutane
Stereoisomers	Same connectivity, different arrangement	Cis/trans-2-butene
Enantiomers	Non-superimposable mirror images	(R)- and (S)-lactic acid
Diastereomers	Not mirror images	(2R,3R)- and (2R,3S)-2,3-dibromopentane

Additional info:

• Periodic table provided for reference in exam; knowledge of atomic numbers and group trends is useful for predicting reactivity and properties.

• Questions cover a range of topics typical for a college-level Organic Chemistry course, including conformational analysis, stereochemistry, radical reactions, and alkyl halide chemistry.